As we saw in the previous article, Laplace's nebular theory gradually fell out of favour with the astronomy community. By the start of the 20th century, some astronomers considered the possibility of a sudden catastrophic event as the cause of our solar system's formation rather than a slow one. To avoid the complications of angular momentum that were caused due to the formation of the Sun, these theories assumed that the Sun already existed and the planets formed later. Although these theories are now lost in antiquity, they led to some important concepts that are still used today.
Chamberlin-Moulton's planetesimals
The first such major theory was proposed by Thomas Chrowder Chamberlin and Forest Ray Moulton around 1900. By this time, the first spiral galaxies had already been discovered but it was thought that these were nebulae within our own galaxy. Chamberlin and Moulton considered the possibility that spiral ‘nebulae’ were formed due to material from a star being pulled out due to another star that passed very close to it.
Think about the active eruptions on the Sun’s surface called solar prominences, but at a much larger scale. The matter from a solar prominence shoots up extremely high before returning back to the Sun, it can go up to a few times the Sun's radius! Their theory proposed that at a time during which the Sun was very active with even larger prominences, a massive star passed very close to the Sun. The gravitational pull of the star disrupted the matter in the prominences and pulled it away from the Sun. This would work like the tidal forces, a prominence towards the star and one away from the star, both would be stretched out like arms giving the resulting shape similar to a spiral nebula. These arms would have denser regions that would break apart and form planetesimals that would merge and form planets.
If this theory seems a bit ad hoc to you, it’s because it certainly is! The probability of a giant star coming close enough to perturb a prominence is minuscule, not even close to the number of spiral nebulae observed even back then. The tidal effects are not symmetric and would lead to a greater prominence on the side facing the star and a smaller one on the other side. However, spiral nebulae were observed to be very symmetrical and could have more than two arms. Lastly, the difference in orbital period between Mercury and Neptune would mean that if they were part of the same prominence, it would break apart almost as soon as it was formed making it even rarer to observe a nebula in this spiral state.
Eventually, the truth about spiral nebulae being completely different galaxies was discovered and the Chamberlin-Moulton theory was abandoned. But another theory arose that was similar in principle and had a firmer theoretical foundation. In 1917, James Jeans realized that prominences were unnecessary if the massive star was close enough.
James Jeans' protoplanets
Jeans proposed that the tidal force would pull out matter from the Sun in the form of a filament. Now hang on, you’d say… isn’t that almost exactly what Chamberlin-Moulton said too? Well, yes… the only difference is that Jeans being a theoretician gave a much more rigorous analysis. He found the type of deformation that a star under tidal forces would experience and showed that these filaments would indeed be produced. He showed that gravitational instability would make the filament break up and found a formula for the minimum mass (now called Jean’s critical mass) required to make the filaments collapse into ‘protoplanets’. These protoplanets would initially be like molten blobs in eccentric orbits and when they got close to the Sun, the same process could repeat on a smaller scale producing ‘protomoons’.
An interesting feature of this theory is that it gave an explanation for the massive gas giants Jupiter and Saturn being roughly in the middle of the solar system. The filament pulled out would have a cigar shape, thicker in the middle when the massive star passed closest and thus pulled out the material at a faster rate. Although the theory suffered the same problem of being highly unlikely to occur, unlike the Chamberlin-Moulton one there was no evidence to check for the likelihood. And as there are hundreds of billions of stars in our galaxy, an unlikely process cannot be considered impossible.
In 1935, Henry Russell put forth a major objection to the theory which was yet again an issue with the angular momentum! He showed that the perihelion distance for material ejected from the Sun could not be greater than the Sun’s radius… which just means that if undisturbed by some external influence, the blobs would fall back into the Sun. At most, even considering that the blobs attracted each other, the maximum distance between the Sun and the blob would be 4 solar radii which is well within the orbit of Mercury.
Lyman Spitzer (1939) used Jean’s critical mass formula to consider the mass required to form Jupiter assuming that the Sun was in the same physical state as today, the material density was the same as the Sun’s average density and the temperature was 10^6 K. The minimum mass he found was 2 x 10^29 kg roughly over 100 times that of Jupiter. This means that even if a Jupiter mass or smaller protoplanet were formed, the pressure would be too great for it to be stable. These two arguments showed that a planet could not possibly be formed according to Jean’s theory.
Despite the failure of these theories, the terms planetesimal and protoplanet are still used in the modern theory of planet formation, although the exact definition has changed. They gave us the idea that planets might not have formed as molten globes of roughly their present size but instead, the process involved the merger of much tinier primordial fragments. The instability of protoplanets formed from solar material made us realize that temperatures for planetary material need to be relatively much cooler - hundreds to thousands of degrees rather than the millions required for nuclear fusion. Jean’s critical mass is also an important concept that is still used in calculations when studying the basics of star formation due to the collapse of giant molecular clouds.
Glossary
solar prominence: These are filament-like structures that are connected to the Sun's surface. They are made of hot plasma and usually fall back to form loops, following the magnetic field of the Sun.
planetesimals: The word is derived from 'planet' and 'infinitesimal'. They are the primordial chunks of rock and ice that coalesced to form planets, with sizes ranging from a few metres to a few km.
protoplanets: The word is derived from 'proto' meaning an early or primitive form and 'planet'. These are formed after the merger of many planetesimals and have sizes similar to the dwarf planets of today.
Jean's critical mass: The mass at which a gaseous cloud can no longer support itself by internal pressure and begins to collapse under its own gravity.
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